Tritiated glucose

In an attempt to detennine the location of glycogen in the liver, could there be any problems of interpretation of the electron microscopic radioautographic images if tritiated glucose were used as the radioactive precursor molecule of glycogen ... 

The solid method did not incorporate this same uncertainty however the tritiated glucose standard used to model the efficiency of the catalytic system employed for pyrolysis cannot be considered truly representative of the nature of the various materials combusted. Therefore an unquantified degree of uncertainty is introduced into calculation of the % conversion of tritium from organic material to water. 

The results of Alizade and Simon (37) also indicate the formation of NADH during the decarboxylation of L-malic acid in Leuconostoc mesen-teroides. During growth on glucose and in the presence of L-malic acid tritiated at the C2 position, tritium was recovered in the C2 position of L-lactic acid, as would be expected from a first decarboxylation. Surprisingly, a significant amount of label was recovered in D-lactic acid which indicated the intermediate formation of tritiated NADH from malic acid. 

The reactions were carried out under standard conditions using the specifically tritiated substrate mixed with uniformly 14C labeled TDP-glucose. Samples were taken at different time intervals and the reaction products were isolated. 14C activity was used to measure the reaction coordinate. The reactions were permitted to proceed until isotopic equilibrium was achieved. 

Non-enzymic aldose-ketose isomerisations that are acid catalysed appear to involve a 1,2-hydride shift. During acid-catalysed rearrangement of glucose to fructose, the label of [2- H]glucose substrate is retained in the [l- H]fructose product, distributed equally between the proR and proS positions." In the reverse sense retention of the label of tritiated fructose in the glucose and mannose products was not complete. Similar observations were made for the xylose-xylulose interconversion." With an appropriate sugar configuration (ribose), even the base-catalysed reaction proceeds partly with retention of label, presumably by the same mechanism as with trioses. 

Our laboratory has studied the stereochemistry of methyl group formation in a number of a, 0 elimination reactions of amino acids catalyzed by pyridoxal phosphate enzymes. The reactions include the conversions of L-serine to pyruvate with tryptophan synthase 02 protein (78) and tryptophanase (79), of L-serine and l-tyrosine with tyrosine phenol-lyase (80), and l-cystine with S-alkylcysteine lyase (81). In the latter study, the stereospecific isotopically labeled L-cystines were obtained enzymatically by incubation of L-serines appropriately labeled in the 3-position with the enzyme O-acetyl serine sulfhy-drase (82). The serines tritiated in the 3-position were prepared enzymatically starting from [l-3H]glucose and [l-3H]mannose by a sequence of reactions of known stereochemistry (81). The cysteines were then incubated with 5-alkyl-cysteine lyase in 2H20 as outlined in Scheme 19. The pyruvate was trapped as lactate, which was oxidized with K2Cr202 to acetate for analysis. Similarly, Cheung and Walsh (71) examined the conversion of D-serine to pyruvate with... 

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